In system applications that require a multiplicity of power supplies, such as for a large number of motor drives, a substantial number of units will invariably fail and require repair. Although power supplies utilize solid state components, each having an inherently high mean time before failure (MTBF), the overall system MTBF is substantially lower due to the large number of discrete components utilized. This is especially so in solid state components that are subjected to high operating temperatures, such as the components in a switching regulator.
Statistically, a percentage of devices will fail regardless of how reliable each component is. This failure rate must be accounted for and either an on site repair facility provided or an inventory maintained to replace the failed power supplies. The first alternative is to maintain an inventory of replacement power supplies that are readily available to replace failed units thus reducing system down time. The failed units are then returned to the manufacturer for repair and reconditioning. This step presents substantial delays and requires a large inventory to be maintained to account for units in repair. This alternative is also a very costly one from both the standpoint of the individual cost of the unit and of the storage cost for maintaining an inventory.
The second alternative is to provide an on-site repair facility for reconditioning the failed power supplies. This is a more feasible alternative in that the main requirement is turnaround time to repair a failed unit and return it to inventory. Sometimes the failure to the power supply is nothing more than an inexpensive component that is readily repairable. The primary costs in repairing a power supply on site are first to determine what the failure is and second to effect the required repair. Normally, the trouble shooting step is readily done whereas repair of the unit presents a more formidable problem.
Components that normally fail in a high voltage power supply are usually those that are subjected to high heat dissipation. These components are firmly attached to the heat sinks and securely wired into the circuit. Due to the operating nature of these components, they are not well suited to employing a modular printed circuit card type of replacement due to high resistance in the conductors. The major portion of the labor in repairing a unit is removing the components from the heat sink rather than disconnecting the wiring. For this reason a modular approach requires removal of both the components and the heat sink, which in the past has consisted of tedious disassembly of mounting hardware.
In view of the above problems, there exists a need for a modular type heat sink that is readily replaceable to facilitate repair in a system application requiring a large number of power supplies.